Avoid printing continuations within import identifiers (#7744)

## Summary

It turns out that _some_ identifiers can contain newlines --
specifically, dot-delimited import identifiers, like:
```python
import foo\
    .bar
```

At present, we print all identifiers verbatim, which causes us to retain
the `\` in the formatted output. This also leads to violating some debug
assertions (see the linked issue, though that's a symptom of this
formatting failure).

This PR adds detection for import identifiers that contain newlines, and
formats them via `text` (slow) rather than `source_code_slice` (fast) in
those cases.

Closes https://github.com/astral-sh/ruff/issues/7734.

## Test Plan

`cargo test`

crates/ruff_python_formatter/src/statement/stmt_import_from.rs

@@ -6,6 +6,7 @@ use ruff_text_size::Ranged;
 use crate::builders::{parenthesize_if_expands, PyFormatterExtensions, TrailingComma};
 use crate::comments::{SourceComment, SuppressionKind};
 use crate::expression::parentheses::parenthesized;
+use crate::other::identifier::DotDelimitedIdentifier;
 use crate::prelude::*;
 
 #[derive(Default)]
@@ -31,7 +32,7 @@ impl FormatNodeRule<StmtImportFrom> for FormatStmtImportFrom {
                     }
                     Ok(())
                 }),
-                module.as_ref().map(AsFormat::format),
+                module.as_ref().map(DotDelimitedIdentifier::new),
                 space(),
                 token("import"),
                 space(),